The present invention relates to labelling and selection of molecules, such as members of a specific binding pair (sbp) able to bind a complementary sbp member of interest, especially though not exclusively a complementary sbp member for which an existing ligand is available. In exemplary embodiments, the present invention relates to selection of antibodies, or polypeptides comprising an antibody antigen binding domain, specific for an antigen of interest for which an existing binding molecule, which may be an antibody, such as a monoclonal antibody, is already available. It involves deposition of a label or reporter molecule, such as biotin-tyramine, on molecules in the vicinity of a xe2x80x9cmarker ligandxe2x80x9d which comprises for example a monoclonal antibody (specific for an antigen of interest) in association with an enzyme which catalyzes such deposition. Molecules labelled in accordance with the present invention may include binding members such as antibodies which bind the same binding target (e.g. antigen) as the marker ligand if such binding members are included in the reaction medium, the target molecule to which the marker ligand binds, which allows for identification and/or purification of unknown antigen targets, and/or other molecules in the vicinity of the binding target and/or the marker ligand when bound to its binding target, e.g. on a cell surface on which the binding target is found, including molecules complexed with the binding target, allowing for identification of novel protein-protein interactions. There are also various advantages in labelling cells or other particles using the present invention, especially when the process is reiterated to augment the extent of labelling. Further aspects and embodiments of the invention are disclosed herein.
Numerous kinds of specific binding pairs are known, as epitomised by the pair consisting of antibody and antigen. Other specific binding pairs are discussed briefly infra and may equally be employed in the various aspects of the present invention disclosed herein. For convenience, however, most of the discussion herein refers to xe2x80x9cantibody xe2x80x9d as the type of (first) specific binding pair (sbp) member whose selection is sought in performance of methods of various embodiments of the invention, xe2x80x9cantigenxe2x80x9d as the complementary (second) sbp member of interest for which specific binding molecules may be sought to be selected and marker ligand as the pre-existing binding molecule known to be able to bind the complementary sbp member of interest. Generally, the marker ligand comprises an antibody antigen binding domain specific for the complementary sbp member of interest (e.g. antigen). Other suitable marker ligands include hormones, cytokines, growth factors, neuropeptides chemokines, enzyme substrates and any other specific binding molecule. Also present is a label or reporter molecule and an enzyme that catalyses binding of the label to other molecules in the vicinity.
Bearing this in mind, the present invention (in some embodiments) can be said to have resulted from the inventors having identified a means to select for antibodies binding to an antigen, e.g. on cell surfaces, other solid supports, or in solution, using a marker ligand for the antigen to guide the recovery of antibodies binding in proximity to the marker ligand. This provides means to label molecules which bind in close proximity to a given defined ligand by transfer of a reporter molecule or label to the binding molecules. The defined ligand occupies a specific epitope on the antigen and generally blocks that particular epitope, and epitopes overlapping it, from binding other antibodies. Thus, antibodies which are selected for are usually those which do not bind to the marker ligand epitope, but are those which bind neighbouring epitopes. Antibodies which bind the same epitope as the original marker ligand may be obtained by an iterative processxe2x80x94using an antibody obtained in one round of the process as a second marker ligand in a further roundxe2x80x94or by using appropriate conditions, as discussed further below.
Signal transfer selection may be used to generate antibodies which bind to the same epitope as the marker ligand by re-iterating the selection procedure. Antibodies selected from the first round of signal transfer selection may be used as new marker ligands for a subsequent round of selection which is carried out in the absence of the original marker ligand. This may be referred to as a step-back selection and may be used to select for antibodies which inhibit the original ligand binding. If the second stage of a step-back selection is carried out in the presence of the original marker ligand antibodies which bind the marker ligand-receptor complex, but not the receptor alone, may be selected. Such antibodies may be ligand agonists or antagonists. Of course, step back selection need not be limited to selection from antibody libraries; any pair of specific binding members can be used in such a procedure.
Antibodies which bind epitopes which are nearest to that bound by the marker ligand have the highest probability of becoming labelled, and the probability of labelling decreases with distance from the marker ligand epitope. Advantageously, the present invention may expedite the purification of such labelled molecules.
Transfer of the biotin tyramine reporter molecule may occur within up to about 25 nm according to experimental results infra. The distance from the binding site of the original marker ligand may be increased by iteration of the signal transfer process, or by adapting the guide molecule by the addition of a spacer between the guide molecule and the enzyme which catalyses the signal transfer. Such a spacer may be a chemical linker, polymer, peptide, polypeptide, rigid bead, phage molecule, or other particle.
Such a spacer may be of any suitable desired length, including about 10-20 nm, about 20-40 nm, about 40-60 nm, about 60-100 nm, about 100 nm or more, such as about 500 nm or more up to about 1 xcexcm or more.
Furthermore, the labelling and subsequent purification of binding molecules specific for antigen of interest which are displayed on the surface of bacteriophage or other biological particles (see e.g. WO92/01047) facilitates recovery of nucleic acid encoding the specific binding molecules. In so-called xe2x80x9cphage displayxe2x80x9d, a binding molecule, e.g. antibody or antibody fragment, peptide or polypeptide, e.g. enzyme, is displayed on the surface of a virus particle which contains nucleic acid encoding the displayed molecule. Following selection of particles that display molecules with the desired binding specificity, the nucleic acid may be recovered from the particles and used to express the specific binding molecules or derivatives thereof, which may then be used as desired.
Other display systems may be used instead of display on filamentous bacteriophage. Such systems include display on whole bacterial cells or modified bacterial surface structures (Osuna et al. Crit. Rev. Microbiol., 1994, 20: 107-116; Lu et al., BioTechnology, 1995, 13: 366-372) and eukaryotic viruses (Boublik et al. BioTechnology, 1995, 13: 1079-1084; Sugiyama et al., FEBS Lett., 1995, L 359: 247-250). Bacteriophage display libraries may be generated using fusion proteins with the gene III protein (e.g. Vaughan et al. Nature Biotechnology, 1996, 14: 309-314), or the major gene VIII coat protein (Clackson and Wells, Trends Biotechnol., 1994, 12: 173-184), or the gene VI protein (Jespers et al., BioTechnology, 1995, 13: 378-382).
Herein it is shown that antibodies binding specifically to a given target antigen, e.g. expressed on the surface of cells, may be selected from a large, diverse phage display library using an existing ligand of the desired antigen to guide the selection. It is also demonstrated that the desired antigen can be purified from the cells by chemical modification of the antigen in a reaction catalysed by the existing ligand. Antibodies to any antigen for which a known ligand exists may be obtained in this way, as may antibodies which bind specifically to the antigen-ligand complex rather than the antigen alone. In addition existing ligands to unknown molecules (e.g. antigens) may be used as markers to guide selection of antibodies to the unknown molecule or purification of the unknown molecule itself. Surface accessible regions of an antigen may be identified by means of their accessibility to labelling, e.g. biotinylation. Biotinylated molecules may be cleaved, e.g. proteolytically if they are peptidyl in nature, and biotinylated fractions detected, e.g. following size fractionation. Furthermore, the labelling of other molecules in the vicinity of the molecule to which the marker ligand binds allows for those other molecules to be identified and/or purified for further study. It also allows for particular moieties on which the binding target appears to be identified and/or purified, for instance one cell type displaying a particular antigen from among a complex mix of different cell types. Determination of the extent of labelling which occurs in the vicinity of a the molecule to which the marker ligand binds may be used to determine the copy number of that molecule, e.g. on a cell surface.
Selection of molecules in accordance with the present invention is not limited to antigens on cell surfaces. For example, complex proteins with multiple domains or subunits may be coated onto a solid support and ligands specific for a particular domain or subunit may be used as marker ligands to guide selection of antibodies to other neighbouring domains or subunits. A domain or subunit may be conjugated, directly or indirectly, to the enzyme (e.g. HRP) and domain-domain or subunit-subunit interactions used to guide selection. This may be termed xe2x80x9cdomain walkingxe2x80x9d. Marker ligands specific for particular epitopes on a protein may also be used to guide the selection away from the marker ligand epitope and to select for binding molecules which bind other epitopes within the radius of labelling (e.g. about 25 nm for biotinylation). This may be termed epitope walking, and example of which is given in Example 8. A xe2x80x9cstep-backxe2x80x9d selection may be carried out (as discussed elsewhere herein), generating a sbp member with the same or overlapping epitope specificity as the original marker ligand.
Techniques of the present invention for selection of molecules, which may be known as xe2x80x9csignal transfer selectionxe2x80x9d, need not be limited to antibody selection; selection from peptide libraries (e.g displayed on phage) may be used to identify peptides with specific binding characteristics for a given protein, which may be any binding domain or type of ligand interaction, not just antibody/epitope. Example 14 illustrates this using peptide libraries to epitope map an antibody (conjugated to HRP) in solution. Libraries or diverse populations of proteins other than antibodies may be displayed on the surface of phage to allow isolation of novel proteins which bind to a protein in proximity to the marker ligand.
Signal transfer selection may also be used to chemically modify a particular cell type possessing a specific antigen to facilitate purification of that cell type from a background of other cells. Signal transfer selection may also be applied to the humanisation of existing monoclonal antibodies since Mab""s which recognise an undefined antigen may be used to target selection of human antibodies with a similar binding capacity. This may involve the marker ligand including the binding domain of a non-human antibody, such as a mouse monoclonal antibody, which may be conjugated directly or indirectly to an enzyme such as HRP. Signal transfer selection may be used to obtain antibodies from a human antibody library displayed on the surface of a suitable virus, such as bacteriophage or retrovirus, or other biological particle, which bind to the same antigen as the pre-existing non-human antibody. Repeating the process (xe2x80x9cstep-backxe2x80x9d) using an antibody obtained in a first performance of the process as the marker ligand in a further performance of the process may be used to obtain human antibodies which bind to the same epitope as the original non-human antibodyxe2x80x94a humanised antibody. Ability of two binding molecules such as antibodies to bind the same epitope may of course be assessed using an appropriate competition assay.
Signal transfer selection may be used to generate two antibodies, or other binding members, which bind adjacent epitopes on the same target molecule. This provides the potential to generate bispecific antibodies (such as xe2x80x9cdiabodiesxe2x80x9d) which may have higher affinities or other desirable biological properties (e.g. neutralising ability) which the individual antibodies alone do not exhibit. Signal transfer selection may also be used with enzyme substrates to direct selection of antibodies which bind enzyme active sites and which may be enzyme inhibitors or activators. Direct biotinylation of the enzyme active site by the substrate may provide a tool to map amino acid residues important in catalysis.
A local supply of hydrogen peroxide or other free radical percursor may be generated by coupling the marker ligand to an enzyme which produces the substrate for the free-radical generating enzyme, such as HRP, for example, glucose oxidase or superoxide dismutase. This enables the local generation of radicalised biotin-tyramine or other label molecule in the vicinity of the free-radical generating enzyme. An active form of free-radical generating enzyme may be generated in response to a binding event, such as the bringing together of two subunits of the enzyme to produce an active enzyme, or bringing together an activator of the enzyme with the enzyme itself. Radicalised label molecule such as biotin-tyramine may be thus generated in response to binding events, which may be between specific cell types, proteins, or other specific binding members.
Specific binding member
This describes a member of a pair of molecules which have binding specificity for one another. The members of specific binding pair may be naturally derived or synthetically produced. One member of the pair of molecules has an area on its surface, or a cavity, which specifically binds to and is therefore complementary to a particular spatial and polar organisation of the other member of the pair of molecules. Thus the members of the pair have the property of binding specifically to each other.
Examples of types of specific binding pairs are antigen-antibody, biotin-avidin/streptavidin, hormone-hormone receptor, receptor-ligand, enzyme-substrate.
Antibody
This describes an immunoglobulin whether natural or partly or wholly synthetically produced. The term also covers any polypeptide or protein having a binding domain which is, or is homologous to, an antibody binding domain. These can be derived from natural sources, or they may be partly or wholly synthetically produced. Examples of antibodies are the immunoglobulin isotypes and their isotypic subclasses; fragments which comprise an antigen binding domain such as Fab, scFv, Fv, dAb, Fd; and diabodies.
It is possible to take monoclonal and other antibodies and use techniques of recombinant DNA technology to produce other antibodies or chimeric molecules which retain the specificity of the original antibody. Such techniques may involve introducing DNA encoding the immunoglobulin variable region, or the complementarity determining regions (CDRs), of an antibody to the constant regions, or constant regions plus framework regions, of a different immunoglobulin. See, for instance, EP-A-184187, GB 2188638A or EP-A-239400. A hybridoma or other cell producing an antibody may be subject to genetic mutation or other changes, which may or may not alter the binding specificity of antibodies produced.
As antibodies can be modified in a number of ways, the term xe2x80x9cantibodyxe2x80x9d should be construed as covering any specific binding member or substance having a binding domain with the required specificity. Thus, this term covers antibody fragments, derivatives, functional equivalents and homologues of antibodies, including any polypeptide comprising an immunoglobulin binding domain, whether natural or wholly or partially synthetic. Chimeric molecules comprising an immunoglobulin binding domain, or equivalent, fused to another polypeptide are therefore included. Cloning and expression of chimeric antibodies are described in EP-A-0120694 and EP-A-0125023.
It has been shown that fragments of a whole antibody can perform the function of binding antigens. Examples of binding fragments are (i) the Fab fragment consisting of VL, VH, CL and CH1 domains; (ii) the Fd fragment consisting of the VH and CH1 domains; (iii) the Fv fragment consisting of the VL and VH domains of a single antibody; (iv) the dAb fragment (Ward, E. S. et al., Nature 341, 544-546 (1989)) which consists of a VH domain; (v) isolated CDR regions; (vi) F(ab"")2 fragments, a bivalent fragment comprising two linked Fab fragments (vii) single chain Fv molecules (scFv), wherein a VH domain and a VL domain are linked by a peptide linker which allows the two domains to associate to form an antigen binding site (Bird et al, Science, 242, 423-426, 1988; Huston et al, PNAS USA, 85, 5879-5883, 1988); (viii) bispecific single chain Fv dimers (PCT/US92/09965) and (ix) xe2x80x9cdiabodiesxe2x80x9d, multivalent or multispecific fragments constructed by gene fusion (WO94/13804; P. Holliger et al Proc. Natl. Acad. Sci. USA 90 6444-6448, 1993).
Diabodies are multimers of polypeptides, each polypeptide comprising a first domain comprising a binding region of an immunoglobulin light chain and a second domain comprising a binding region of an immunoglobulin heavy chain, the two domains being linked (e.g. by a peptide linker) but unable to associate with each other to form an antigen binding site: antigen binding sites are formed by the association of the first domain of one polypeptide within the multimer with the second domain of another polypeptide within the multimer (WO94/13804).
Antigen binding domain
This describes the part of an antibody which comprises the area which specifically binds to and is complementary to part or all of an antigen. Where an antigen is large, an antibody may only bind to a particular part of the antigen, which part is termed an epitope. An antibody antigen binding domain may be provided by one or more antibody variable domains. Preferably, an antigen binding domain comprises an antibody light chain variable region (VL) and an antibody heavy chain variable region (VH).
Specific
This refers to the situation in which one member of a specific binding pair will not show any significant binding to molecules other than its specific binding partner (e.g. an affinity of about 1000xc3x97 worse). The term is also applicable where eg an antigen binding domain is specific for a particular epitope which is carried by a number of antigens, in which case the specific binding member carrying the antigen binding domain will be able to bind to the various antigens carrying the epitope.
Functionally equivalent variant form
This refers to a molecule (the variant) which although having structural differences to another molecule (the parent) retains some significant homology and also at least some of the biological function of the parent molecule, e.g. the ability to bind a particular antigen or epitope. Variants may be in the form of fragments, derivatives or mutants. A variant, derivative or mutant may be obtained by modification of the parent molecule by the addition, deletion, substitution or insertion of one or more amino acids, or by the linkage of another molecule. These changes may be made at the nucleotide or protein level. For example, the encoded polypeptide may be a Fab fragment which is then linked to an Fc tail from another source. Alternatively, a marker such as an enzyme, flourescein, etc, may be linked.
Marker ligand
This refers to one member of a specific binding pair able to bind complementary sbp member. In embodiments of the present invention, it is used to guide catalysis of label or reporter molecule deposition at and around its site of binding to the complementary other member of the specific binding pair.
According to a first aspect of the present invention there is provided a method of labelling molecules, the method including
providing in a common medium:
a label molecule;
a ligand (xe2x80x9cfirst marker ligandxe2x80x9d) able to bind a second member of a specific binding pair (sbp);
a said second sbp member;
an enzyme able to catalyse binding of said label molecule to other molecules, said enzyme being associated with said first marker ligand;
causing or allowing binding of-said first marker ligand to said second sbp member; and
causing or allowing binding of said label molecule to other molecules in the vicinity of said first marker ligand bound to said second sbp member.
A first member of a specific binding pair, such as an antibody, may be included, or a diverse population of such first sbp members including one or more which bind the second sbp member. Molecules to which the label molecule binds may include a sbp member (xe2x80x9cfirst sbp memberxe2x80x9d) which binds said second sbp member.
Molecules to which the label molecule binds may include a sbp member (xe2x80x9cfirst sbp memberxe2x80x9d) which binds a molecule in the vicinity of said second sbp member, as discussed further infra.
In preferred embodiments of the invention the first sbp member is a polypeptide comprising an antibody antigen binding domain, and the second, complementary sbp member is antigen. The marker ligand may be a polypeptide comprising an antibody antigen binding domain, such as a monoclonal antibody or cloned scFv, Fab or other antibody fragment.
In a preferred embodiment of the present invention, the first member of the-specific binding pair is included and is labelled by binding of the label molecule. This allows identification and/or isolation of-target molecules such as antibodies able to bind a substance of interest, such as antigen. (The term xe2x80x9ctarget moleculesxe2x80x9d may be used to refer to molecules the identification of which is the object of the person skilled in the art operating the invention.) Such isolation may be facilitated if the label itself is a member of a specific binding pair. A preferred label exemplified herein is biotin, able specifically to bind avidin and streptavidin. Also exemplified is the use of light-activatible streptavidin as the label.
Following binding of a sbp member label such as biotin to a target sbp member (e.g. antibody), specific binding of the label to its complementary sbp member (e.g. streptavidin in the case of biotin labelling) may be used in isolation of the target sbp member. For instance, streptavidin-coated magnetic beads may be added to the medium or milieu, allowing streptavidin-biotin binding to take place, then extracted using a magnet. Sbp members labelled with biotin may then be recovered from the beads.
Other suitable labels include photo-reactive compounds such as N-[N-4-azido-tetraflurobenzoyl)-biocytinyloxy]-succinimide, or photoreactive crosslinking agents such as sulfor-SANPAH or SAND (sulfosuccinimidyl 2-[m-azido-o-nitrobenzamido]-ethyl-1,3-dithiopropionate) in combination with streptavidin or biotin. Conveniently, biotin or other label is conjugated to tyramine, whose covalent binding to peptide molecules is catalysed by oxygen free radicals generated by hydrogen peroxidase in the presence of hydrogen peroxide. Instead of biotin-tyramine, labelling in performance of the present invention may employ other forms of modified tyramine including fluoresceinated tyramine or other free radical reagents, such as p-hydroxyphenylpropionyl-biocytin and biotynil-coumarin galactose. Labels such as biotin (e.g. as biotin-tyramine) may be preferred over photo-reactive labels, e.g. because of ease of handling, though Example 10 below demonstrates operation of the present invention using a label whose binding is light-activated, i.e. SAND linked to streptavidin. An advantage of using a light-activatable label, such as streptavidin-SAND, is the distance over which this label can be deposited. The linker between the streptavidin and SAND is 1.8 nm so the proximity within which the streptavidin is deposited is up to a maximum of about 1.8 nm, compared with a radius of up to about 25 nm of biotinylation which is obtainable with biotin-tyramine.
The enzyme that catalyses binding of the label molecule to other molecules may be associated with the marker ligand by any suitable means available in the art. It may be conjugated directly, e.g. via a peptide bond (in which case a fusion protein comprising marker ligand and enzyme may be produced by expression from encoding nucleic acid), or by chemical conjugation of the marker ligand and enzyme, or indirectly. Indirect conjugation of enzyme and marker ligand may conveniently be achieved using a further binding molecule that forms a specific binding pair with the marker ligand. For example, the marker ligand may be a mouse monoclonal antibody, or may comprise a mouse antibody sequence, and the enzyme may be provided conjugated to an anti-mouse antibody or antibody antigen binding domain (e.g. as a fusion protein). Binding of anti-mouse antibody to the mouse monoclonal, itself binding the antigen of interest (second sbp member), brings the conjugated enzyme into close proximity with the antigen and any molecules in the medium or milieu able to bind the antigen (e.g. target antibodies), allowing the enzyme to catalyse labelling of such molecules (e.g. target antibodies) and/or the antigen. Labelled molecules may be identified and/or isolated for investigation and/or use.
As mentioned already, the first sbp member when provided in the reaction milieu may be one of a diverse population of that type of sbp member with different binding specificities. Such a population may be provided by expression from a genetically diverse repertoire of nucleic acid sequences. In the case of antibody antigen binding domains, these may be provided by expression from a repertoire of rearranged or unrearranged immunoglobulin sequences from an organism (preferably human) which has or has not been immunised with the antigen of interest. A repertoire of sequences encoding antibody antigen binding domains (VH and/or VL) may additionally or alternatively be provided by any of artificial rearrangement of V, J and D gene segments, mutation in vitro or in vivo, in vitro polynucleotide synthesis and/or any other suitable technique available in the art. Suggested references include Vaughan et al., (1996) Nature Biotechnology 14: 309-314; Griffiths et al., (1993) EMBO J. 12: 725-734.
Conveniently, a diverse population of binding molecules is provided displayed on the surface of a biological particle such as a virus, e.g. bacteriophage, each particle containing nucleic acid encoding the binding molecule displayed on its surface. WO92/01047 discloses in detail various formats for xe2x80x9cphage displayxe2x80x9d of polypeptides and peptide binding molecules, such as antibody molecules, including scFv, Fab and Fv fragments, and enzymes, both monomeric and polymeric. Following labelling of phage displaying a target sbp member able to bind complementary sbp member of interest, and isolation of these from the reaction medium or milieu as discussed, nucleic acid may be recovered from phage particles. This nucleic acid may be sequenced if desired.
Other display Systems, e.g. on bacterial cells or retroviruses, are applicable, as has been mentioned already.
The nucleic acid taken from the particle, or its nucleotide sequence, may be used to provide nucleic acid for production of the encoded polypeptide or a fragment or derivative thereof in a suitable expression system, such as a recombinant host organism. A derivative may differ from the starting polypeptide from which it is derived by the addition, deletion, substitution or insertion of amino acids, or by the linkage of other molecules to the encoded polypeptide. These changes may be made at the nucleotide or protein level. For example the encoded polypeptide may be a Fab fragment which is then linked to an Fc tail from another source. Alternatively markers such as enzymes, flouresceins etc may be linked to eg Fab, scFv fragments.
Systems for cloning and expression of a polypeptide in a variety of different host cells are well known. Suitable host cells include bacteria, mammalian cells, yeast and baculovirus systems. Mammalian cell lines available in the art for expression of a heterologous polypeptide include Chinese hamster ovary cells, HeLa cells, baby hamster kidney cells and many others. A common, preferred bacterial host is E. coli. 
Suitable vectors can be chosen or constructed, containing appropriate regulatory sequences, including promoter sequences, terminator fragments, polyadenylation sequences, enhancer sequences, marker genes and other sequences as appropriate. Vectors may be plasmids, viral e.g. xe2x80x2phage, or phagemid, as appropriate. For further details see, for example, Molecular Cloning: a Laboratory Manual: 2nd edition, Sambrook et al., 1989, Cold Spring Harbor Laboratory Press. Many known techniques and protocols for manipulation of nucleic acid, for example in preparation of nucleic acid constructs, mutagenesis, sequencing, introduction of DNA into cells and gene expression, and analysis of proteins, are described in detail in Short Protocols in Molecular Biology, Second Edition, Ausubel et al. eds., -John Wiley and Sons, 1992. The disclosures of Sambrook et al. and Ausubel et al. are incorporated herein by reference.
The expression end product may be used to prepare a composition comprising the expression end product or a derivative thereof and optionally one or more further components such as a pharmaceutically acceptable vehicle, carrier or excipient, which may for example be used as a therapeutic or prophylactic medicament or a diagnostic product.
In some embodiments of the present invention, the second sbp member (to which the marker ligand bindsxe2x80x94e.g. antigen) is labelled. This is useful if the target molecule is an unknown antigen/receptor for the known marker ligand (e.g. monoclonal antibody or the natural ligand for the antigen/receptor). In such case, the first sbp member may be omitted from the reaction medium or milieu. Following labelling of the second sbp member it may be identified and/or isolated in accordance with procedures disclosed herein.
According to a further aspect of the present invention there is provided reaction medium or milieu containing:
a member of said specific binding pair;
a label molecule;
a ligand (xe2x80x9cmarker ligandxe2x80x9d) able to bind said sbp member;
an enzyme able to catalyse binding of said label molecule to other molecules, said enzyme being associated with said marker ligand;
as provided in methods according to the invention. A further sbp member (designated xe2x80x9cfirstxe2x80x9d) may be present, in which case the marker ligand is able to bind complementary xe2x80x9csecondxe2x80x9d sbp member.
A further aspect of the present invention provides a sbp member identified as having ability to bind complementary sbp member of interest and/or isolated using a method as disclosed herein, including a receptor or ligand identified and/or isolated as disclosed, and compositions comprising such an identified and/or isolated sbp member and nucleic acid encoding the identified and/or isolated sbp member.
The present invention generally provides for any specific binding member identified by virtue of its ability to bind to complementary sbp member in close proximity (e.g. less than about 25 nm, and possibly less than about 20 nm, less than about 15 nm, less than about 10 nm, about 5-10 nm or about 5 nm) to an existing defined ligand, which may be termed a xe2x80x9cmarker ligandxe2x80x9d and is used to guide catalysis of reporter molecule deposition on to the specific binding member.
The invention also provides for the use of the methods and means provided herein for the selection of phage-displayed sbp members, e.g. antibodies, peptides or proteins, also the selection or identification of unknown receptors using a known ligand, either by directed labelling of the receptor, or by production of an antibody against the receptor, followed by immuno-purification.
The invention also provides for the use of signal transfer selection in an iterative manner, i.e. using one or more sbp members selected in a cycle to select for further sbp members. This may be used to select sbp members which are capable of acting as antagonists or agonists to the original marker ligand used in the first stage of the selection.
Cell-surface or other receptors may be identified in a process according to the present invention by conjugating a ligand for the uncharacterised receptor (e.g. the natural of the receptor) with an enzyme able to catalyse binding of the label molecule. Binding of the ligand to the receptor, e.g. on cells expressing it, may then be carried out in the presence or absence of sbp members, such as antibodies, particularly a library of sbp members, e.g. displayed on phage, and the label molecule. The natural ligand may transfer the signal molecule directly onto the unknown receptor. Labelled receptor may then be directly purified, e.g. from a cell extract, and may be protein sequenced. In the presence of the sbp members, e.g. a library of antibodies displayed on phage, signal transfer will generate labelled sbp members which are able to bind the receptor. These may then be used to generate purified receptor by affinity purification.
The invention also provides for the use of such processes to identify unknown ligands for known receptors, either by directed labelling of the ligand, or by production of an antibody directed against the ligand followed by immuno-purification.
Further provided by the invention is the use of signal transfer selection to guide the selection of antibodies to a given epitope, domain or subunit of a protein or complex by an existing ligand or antibody which recognises a neighbouring epitope, domain or subunit. Existing sbp""s (e.g. monoclonal antibodies) to a defined but perhaps undesirable epitope, subunit or region of a protein complex may be conjugated to an enzyme capable of catalysing binding of the label molecule to other molecules. These conjugated sbp""s may then be used to direct signal transfer of the label to other sbp members, e.g. antibodies (e.g. on phage), binding to the same antigen but at non-identical, non-overlapping, but neighbouring epitopes which may be on adjacent subunits of a protein, or on adjacent regions of a protein complex.
Signal transfer selection may be used to obtain antibodies or other binding molecules which bind to the same epitope as the marker ligand. For example, sub-saturating amounts of the marker ligand may be added to a mutlimeric protein and the marker ligand may then direct selection of binding specificities recognising the same epitope as the marker ligand, but on a neighbouring subunit, or copy of the multimer. The marker ligand may be capable of labelling binding species which bind to the same epitope if labelling occurs concomitantly with the marker ligand being competed off the target protein by the species which is being selected for.
Another application of the process is that of selecting for antibodies or other ligands which bind to a particular cell structure or cell type.
Further aspects of the present invention arise from the gene cloning work described in Example 16. Encoding nucleic acid, isolated polypeptides, specific binding molecules for the polypeptide and other molecules which interact with the polypeptide, particularly those which modulate its function, e.g. interfere with its association with CC-CKR5 and/or other polypeptide in the vicinity of CC-CKR5 on the surface of CD4+ cells, other molecules which interact with the polypeptide, and methods and uses of these are all provided by the present invention.
Nucleic acid according to this aspect of the present invention may include or consist essentially of a nucleotide sequence encoding a polypeptide which includes an amino acid sequence shown in FIG. 8.
The coding sequence may be that shown in FIG. 8, or it may be a mutant, variant, derivative or allele of the sequence shown. The sequence may differ from that shown by a change which is one or more of addition, insertion, deletion and/or substitution of one or more nucleotides of the sequence shown. Changes to a nucleotide sequence may result in an amino acid change at the protein level, or not, as determined by the genetic code.
Thus, nucleic acid according to the present invention may include a sequence different from the sequence shown in FIG. 8 yet encode a polypeptide with-the same amino acid sequence. The polypeptide may include a sequence of about 60 contiguous amino acids from FIG. 8, more preferably about 70 contiguous amino acids, more preferably about 80. An amino acid sequence from the second reading frame may be preferred. A stop codon occurs in this frame at nucleotide 251, so in a preferred embodiment the polypeptide includes a contiguous sequence of amino acids encoded by the nucleotide sequence of the second reading frame of FIG. 8 up to said stop codon. Usually, additional amino acids are included N-terminal to the amino acid sequence shown.
On the other hand, the encoded polypeptide may include an amino acid sequence which differs by one or more amino acid residues from the relevant amino acid sequence shown in FIG. 8. Nucleic acid encoding a polypeptide which is an amino acid sequence mutant, variant, derivative or allele of a sequence shown in FIG. 8 is further provided by the present invention.
Nucleic acid encoding such a polypeptide may show at the nucleotide sequence and/or encoded amino acid level greater than about 50% homology with the relevant coding/amino acid sequence shown in FIG. 8, greater than about 60% homology, greater than about 70% homology, greater than about 80% homology, greater than about 90% homology or greater than about 95% homology.
As is well-understood, homology at the amino acid level is generally in terms of amino acid similarity or identity. Similarity allows for xe2x80x9cconservative variationxe2x80x9d, such as substitution of one hydrophobic residue such as isoleucine, valine, leucine or methionine for another, or the substitution of one polar residue for another, such as arginine for lysine, glutamic for aspartic acid, or glutamine for asparagine. Similarity may be as defined and determined by the TBLASTN program, of Altschul et al. (1990) J. Mol. Biol. 215: 403-10, which is in standard use in the art. Homology may be over the full-length of the relevant amino acid sequence of FIG. 8, or may more preferably be over a contiguous sequence of about 20, 25, 30, 40, 50, 60, 70, 80 or more amino acids, compared with the relevant amino acid sequence of FIG. 8.
At the nucleic acid level, homology may be over the full-length or more preferably by comparison with the a contiguous nucleotide coding sequence within the sequence of FIG. 8 of about 50, 60, 70, 80, 90, 100, 120, 150, 180, 210, 240 or more nucleotides.
Generally, nucleic acid according to the present invention is provided as an isolate, in isolated and/or purified form, or free or substantially free of material with which it is naturally associated, such as free or substantially free of nucleic acid flanking the gene in the human genome, except possibly one or more regulatory sequence(s) for expression. Nucleic acid may be wholly or partially synthetic and may include genomic DNA, cDNA or RNA. Where nucleic acid according to the invention includes RNA, reference to the sequence shown should be construed as reference to the RNA equivalent, with U substituted for T.
Nucleic acid sequences encoding all or part of the gene and/or its regulatory elements can be readily prepared by the skilled person using the information and references contained herein and techniques known in the art (for example, see Sambrook, Fritsch and Maniatis, xe2x80x9cMolecular Cloning, A Laboratory Manual, Cold Spring Harbor Laboratory Press, 1989, and Ausubel et al, Short Protocols in Molecular Biology, John Wiley and Sons, 1992).
The sequence information provided in FIG. 8 enables cloning of the full-length human coding sequence. The present invention provides a method of obtaining nucleic acid of interest, the method including hybridisation of a probe having the sequence shown in FIG. 8 or a complementary sequence, or a suitable fragment of either, to target nucleic acid. Hybridisation is generally followed by identification of successful hybridisation and isolation of nucleic acid which has hybridised to the probe, which may involve one or more steps of PCR. The nucleic acid sequences provided herein readily allow the skilled person to design PCR primers for amplification of the full-length sequence.
Nucleic acid according to the present invention is obtainable using one or more oligonucleotide probes or primers designed to hybridise with one or more fragments of the nucleic acid sequence shown in FIG. 8 particularly fragments of relatively rare sequence, based on codon usage or statistical analysis. A primer designed to hybridise with a fragment of the nucleic acid sequence shown in FIG. 8 may be used in conjunction with one or more oligonucleotides designed to hybridise to a sequence in a cloning vector within which target nucleic acid has been cloned, or in so-called xe2x80x9cRACExe2x80x9d (rapid amplification of cDNA ends) in which cDNA""s in a library are ligated to an oligonucleotide linker and PCR is performed using a primer which hybridises with the sequence shown in FIG. 8 and a primer which hybridises to the oligonucleotide linker.
Such oligonucleotide probes or primers, as well as the full-length sequence (and mutants, alleles, variants and derivatives) are also useful in screening a test sample containing nucleic acid for the presence of alleles, mutants and variants, with diagnostic and/or prognostic implications.
Nucleic acid isolated and/or purified from one or more cells (e.g. human) or a nucleic acid library derived from nucleic acid isolated and/or purified from cells (e.g. a cDNA library derived from mRNA isolated from the cells), may be probed under conditions for selective hybridisation and/or subjected to a specific nucleic acid amplification reaction such as the polymerase chain reaction (PCR), as discussed.
In the context of cloning, it may be necessary for one or more gene fragments to be ligated to generate a full-length coding sequence. Also, where a full-length encoding nucleic acid molecule has not been obtained, a smaller molecule representing part of the full molecule, may be used to obtain full-length clones. Inserts may be prepared from partial cDNA clones and used to screen cDNA libraries. The full-length clones isolated may be subcloned into expression vectors and activity assayed by transfection into suitable host cells, e.g. with a reporter plasmid.
Those skilled in the art are well able to employ suitable conditions of the desired stringency for selective hybridisation, taking into account factors such as oligonucleotide length and base composition, temperature and so on.
On the basis of amino acid sequence information, oligonucleotide probes or primers may be designed, taking into account the degeneracy of the genetic code, and, where appropriate, codon usage of the organism from the candidate nucleic acid is derived. An oligonucleotide for use in nucleic acid amplification may have about 10 or fewer codons (e.g. 6, 7 or 8), i.e. be about 30 or fewer nucleotides in length (e.g. 18, 21 or 24). Generally specific primers are upwards of 14 nucleotides in length, but not more than 18-20. Those skilled in the art are well versed in the design of primers for use processes such as PCR. Various techniques for synthesizing oligonucleotide primers are well known in the art, including phosphotriester and phosphodiester synthesis methods.
A further aspect of the present invention provides an oligonucleotide or polynucleotide fragment of the nucleotide sequence shown in FIG. 8, or a complementary sequence, in particular for use in a method of obtaining and/or screening nucleic acid. Some preferred-oligonucleotides have a sequence shown in FIG. 8 or a sequence which differs from any of the sequences shown by addition, substitution, insertion or deletion of one or more nucleotides, but preferably without abolition of ability to hybridise selectively with nucleic acid with the sequence shown in FIG. 8, that is wherein the degree of homology of the oligonucleotide or polynucleotide with one of the sequences given is sufficiently high.
Nucleic acid according to the present invention may be used in methods of gene therapy, for instance in treatment of individuals with the aim of preventing or curing (wholly or partially) a disease. This may ease one or more symptoms of the disease.
A convenient way of producing a polypeptide according to the present invention is to express nucleic acid encoding it, by use of the nucleic acid in an expression system.
Accordingly, the present invention also encompasses a method of making a polypeptide (as disclosed), the method including expression from nucleic acid encoding the polypeptide (generally nucleic acid according to the invention). This may conveniently be achieved by growing a host cell in culture, containing such a vector, under appropriate conditions which cause or allow expression of the polypeptide. Polypeptides may also be expressed in in vitro systems, such as reticulocyte lysate.
Systems for cloning and expression of a polypeptide in a variety of different host cells are well known. Suitable host cells include bacteria, eukaryotic cells such as mammalian and yeast, and baculovirus systems. Mammalian cell lines available in the art for expression of a heterologous polypeptide include Chinese hamster ovary cells, HeLa cells, baby hamster kidney cells, COS cells and many others. A common, preferred bacterial host is E. coli. 
Nucleic acid may be introduced into a host cell and this may be followed by causing or allowing expression from the nucleic acid, e.g. by culturing host cells (which may include cells actually transformed although more likely the cells will be descendants of the transformed cells) under conditions for expression of the gene, so that the encoded polypeptide is produced. If the polypeptide is expressed coupled to an appropriate signal leader peptide it may be secreted from the cell into the culture medium. Following production by expression, a polypeptide may be isolated and/or purified from the host cell and/or culture medium, as the case may be, and subsequently used as desired, e.g. in the formulation of a composition which may include one or more additional components, such as a pharmaceutical composition which includes one or more pharmaceutically acceptable excipients, vehicles or carriers (e.g. see below).
The skilled person can use the techniques described herein and others well known in the art (for which see e.g. the Sambrook and Ausubel references cited herein) to produce large amounts of polypeptide, or fragments or active portions thereof, for use as pharmaceuticals, in the developments of drugs and for further study into its properties and role in vivo.
Thus, a further aspect of the present invention provides a polypeptide which includes an amino acid sequence shown in FIG. 8 as discussed, which may be in isolated and/or purified form, free or substantially free of material with which it is naturally associated, such as other polypeptides or such as human polypeptides other than polypeptide or (for example if produced by expression in a prokaryotic cell) lacking in native glycosylation, e.g. unglycosylated.
Polypeptides which are amino acid sequence variants, alleles, derivatives or mutants are also provided by the present invention, as has been discussed. Preferred such polypeptides have function, that is to say have one or more of the following properties: immunological cross-reactivity with an antibody reactive with a polypeptide for which the sequence is given in FIG. 8; sharing an epitope with a polypeptide for which the amino acid sequence is shown in FIG. 8 (as determined for example by immunological cross-reactivity between the two polypeptides.
The present invention also includes active portions, fragments, derivatives and functional mimetics of the polypeptides of the invention. A fragment of the polypeptide may be a stretch of amino acid residues of at least about five to seven contiguous amino acids, often at least about seven to nine contiguous amino acids, typically at least about nine to 13 contiguous amino acids and, most preferably, at least about 20 to 30 or more contiguous amino acids. Fragments of the polypeptide sequence antigenic determinants or epitopes useful for raising antibodies to a portion of the amino acid sequence.
A polypeptide, peptide fragment, allele, mutant or variant according to the present invention may be used in phage display or other technique (e.g. involving immunisation) in obtaining specific antibodies. Antibodies are useful in purification and other manipulation of polypeptides and peptides, diagnostic screening and therapeutic contexts.
The provision of the novel polypeptides enables for the first time the production of antibodies able to bind it specifically, and by procedures other than the signal transfer selection which led to its identification and the isolation of antibody CD4E1 as described in Example 16. Accordingly, a further aspect of the present invention provides an antibody able to bind specifically to a polypeptide including a sequence given in FIG. 8.
Such antibodies may be obtained by selection on peptides or proteins including amino acid sequences of FIG. 8, e.g. using phage display libraries as in WO92/01047, or by using such peptides or proteins to immunise animals and obtain monoclonal antibodies or polyclonal antisera.
Antibodies identified, e.g. by phage display, may then be used to identify further proteins, e.g. receptor molecules, which may be complexed with the protein including the amino acid sequence of FIG. 8, using techniques of signal transfer selection as disclosed herein.
cDNA expression libraries, for example displayed on phage, may be used in conjunction with signal transfer selection to identify ligands which bind molecules, such as receptors, in the vicinity of protein including the amino acid sequence of FIG. 8. An antibody, e.g. with a myc tag, may bind to the protein on the surface of CD4 lymphocytes. the phage-displayed cDNA expression library may be added, followed by the antibody 9E10 (which binds to the myc tag) conjugated to HRP. Adition of biotin-tyramine would then lead to the labelling of molecules in the vicinity of the antibody, including phage expressing receptor ligands. The antibody CD4E1 would be suitable for this.
The polypeptides, antibodies, peptides and nucleic acid of the invention may be formulated-in a composition. Such a composition may include, in addition to one of the above substances, a pharmaceutically acceptable excipient, carrier, buffer, stabiliser or other materials well known to those skilled in the art. Such materials should be non-toxic and should not interfere with the efficacy of the active ingredient. The precise nature of the carrier or other material may depend on the route of administration, e.g. oral, intravenous, cutaneous or subcutaneous, nasal, intramuscular, intraperitoneal routes.
A polypeptide according to the present invention may be used in screening for molecules which affect or modulate its activity or function, including ability to interact or associate with another molecule, such as CC-CKR5 or other molecule, e.g. on the surface of CD4+ cells. Such molecules may be useful in a therapeutic (possibly including prophylactic) context.
A method of screening for a substance which modulates activity of a polypeptide may include contacting one or more test substances with the polypeptide in a suitable reaction medium, testing the activity of the treated polypeptide and comparing that activity with the activity of the polypeptide in comparable reaction medium untreated with the test substance or substances. A difference in activity between the treated and untreated polypeptides is indicative of a modulating effect of the relevant test substance or substances.
Combinatorial library technology provides an efficient way of testing a potentially vast number of different substances for ability to modulate activity of a polypeptide. Such libraries and their use are known in the art. The use of peptide or protein libraries may be preferred.
As an alternative to using signal transfer selection to identify molecules which interact with protein including an amino acid sequence shown in FIG. 8, test substances may be screened for ability to interact with the polypeptide, e.g. in a two-hybrid system (which requires that both the polypeptide and the test substance can be expressed, e.g. in a cell such as a yeast or mammalian cell, from encoding nucleic acid). This may be used as a coarse screen prior to testing a substance for actual ability to modulate activity of the polypeptide. The screen may be used to screen test substances for binding to a specific binding partner, to find mimetics of polypeptide, e.g. for testing as anti-tumour therapeutics. Two-hybrid screens may be used to identify a substance able to modulate, e.g interfere with, interaction between two polypeptides or peptides.
The two-hybrid screen assay format is described by Fields and Song, 1989, Nature 340; 245-246. This type of assay format can be used in both mammalian cells and in yeast. Various combinations of DNA binding domain and transcriptional activation domain are available in the art, such as the LexA DNA binding domain and the VP60 transcriptional activation domain, and the GAL4 DNA binding domain and the GAL4 transcriptional activation domain. Suitable fusion constructs are produced for expression within the assay system. When screening for a susbstance able to modulate an interaction between two components, test substances (e.g. in a combinatorial peptide library) may be expressed from a third construct.
Following identification of a substance which modulates or affects polypeptide activity and/or its ability to interact with or associate with another molecule, the substance may be investigated further. Furthermore, it may be manufactured and/or used in preparation, i.e. manufacture or formulation, of a composition such as a medicament, pharmaceutical composition or drug. These may be administered to individuals.
Thus, the present invention extends in various aspects not only to a substance identified using a nucleic acid molecule as a modulator of polypeptide activity, in accordance with what is disclosed herein, but also a pharmaceutical composition, medicament, drug or other composition comprising such a substance, a method comprising administration of such a composition to a patient, e.g. for treatment (which may include preventative treatment) of cancer, use of such a substance in manufacture of a composition for administration, e.g. for treatment of cancer, and a method of making a pharmaceutical composition comprising admixing such a substance with a pharmaceutically acceptable excipient, vehicle or carrier, and optionally other ingredients.
Further aspects of the invention and embodiments will be apparent to those skilled in the art. All documents mentioned herein are incorporated by reference. In order that the present invention may be fully understood the following examples are provided by way of exemplification only and not by way of limitation.